Multilayer laminated polyethylene copolymer-polyamide film

ABSTRACT

A heat sealable laminated film including at least a layer of a polyamide polymer, a layer of a blend of polyethylene and a zincneutralized ionic copolymer, and an adherent layer of a zincneutralized ionic copolymer of ethylene and an Alpha ,Bethylenically unsaturated carboxylic acid. Preferred respective materials are polycaproamide and a zinc-neutralized ionic copolymer of ethylene and methacrylic acid.

United States Patent 1 Goehring et al. 1

[451 Feb. 12, 1974 MULTILAYER LAMINATED POLYETHYLENE COPOLYMER-POLYAMIDEFILM [75] Inventors: Clifford Clayton Goehring,

Princeton; Arthur Clifford Hart, Jr.,

Chester; Derek Wooldridge, Princeton, all of NJ.

[73] Assignee: American Can Company,

Greenwich, Conn.

22 Filed: July 6, 1971 [21] Appl. No.: 160,145

[52] US. Cl 161/227, 99/171 H, 99/171 LP, 156/244, 156/334, 161/254,260/78.5 T [51] Int. Cl ..'B32b 27/34, B32b 27/08 [58] Field ofSearch..... 99/171 H, 171 LP; 156/334; 161/227, 254; 260/785 T [56]References Cited UNITED STATES PATENTS 3,423,231 l/1969 Lutzmann117/68.5

3,355,319 11/1967 Rees 117/122 3,697,368 10/1972 Bhuta et a1. 161/227Primary ExaminerAlfred L. Leavitt Assistant Examiner-Robert A. DawsonAttorney, Agent, or FirmRobert P. Auber; Paul R. Audet; George P.Ziehmer 5 7] ABSTRACT 8 Claims, No Drawings MULTILAYER LAMINATEDPOLYETHYLENE COPOLYMER-POLYAMIDE FILM BACKGROUND OF THE INVENTION Thisinvention relates to laminated films and analogous sheetlike structuresfor wrapping, packaging and related purposes. More particularly, theinvention relates to improved transparent plastic films which are heatsealable and which have high thermal stability and a high resistance topermeation of water and oxygen.

It is well known that laminates of nylon and polyethylene are widelyused for packaging various products. The materials have been foundespecially suitable for foodstuff-containing packages such as bags orpouches subjectable to widely varying temperatures and conditions. Forexample, they are especially suitable for boilin-the-bag food packages.

Nylon is used as an outer surface for such packages and is suitablebecause it is an oxygen barrier, it has a high melting point and isstrong and clear. Polyethylene is used as an inner surface and issuitable because it is easily heat scalable, it is moisture impermeableand is relatively chemically inert to many foodstuffs.

Disadvantages of nylon mainly are its high cost, moisture permeability,poor heat sealability and its tendency to be slow to recover uponphysical deformation. These disadvantages, however, are largely offsetby polyethylenes low cost,v low permeability, excellent heatsealability, and its flexibility and quick recovery.

Although the complementary characteristics of nylon and polyethylenemake them suitable for use as laminates, especially for foodstuffcontaining packages, the materials are somewhat incompatible becausethey often are considerably difficult to initially bond together. Thisis because of their differences in physical and chemical structures.Also once joined, the materials often separate upon physicaldeformation. This is because of the materials aforementioned differingdeformation and recovery characteristics.

A number of successful techniques have been developed to overcome thesebonding difficulties. These techniques include chemically, electricallyor otherwise treating the surface of the polyethylene. These techniquesare summarized in U.S. Pat. No. 3,423,231, filed on May 20, 1965. Asstated therein, surface treatments are expensive, complex and oftendangerous. In addition, their effectiveness is often unsatisfactory.

Another technique which does not involve chemical, electrical or othersurface treatments is to incorporate a layer of adhesive between thenylon and polyethylene. For example, the aforementioned referencediscloses a nylon-polyethylene laminate bonded by a layer ofethylene-acrylic acid copolymeradhesive. Such an adhesive is said tosatisfactorily bond nylonpolyethylene laminates under normal conditionsof handling and use. It has been found that the adhesive isunsatisfactory for nylon-polyethylene laminate constructions such asboil-in-the-bag packages which are subjected to higher than normal usetemperatures. These temperatures, e.g., boil-in-the-bag temperatures,are often over 200F, and at such temperatures, delamination occurs andthe ethylene-acrylic acid copolymer adhesive tends to soften and creepexcessively.

The laminate construction of this invention utilizing a zinc-neutralizedionic copolymer adhesive is not made by chemical, electrical or othersurface treatment techniques yet it performs satisfactorily andmaintains sufficient cohesion especially at boil-in-the-bagtemperatures. The laminate is dimensionally stable in cold environmentsand, in addition, has a high resistance to oxygen, moisture, water, andvarious foodstuffs. For certain applications, it has been found thateven greater than the aforementioned satisfactory cohesion, stabilityand resistance to varied environments can be obtained at boiling watertemperatures when the polyolefin hydrocarbon polymer layer comprises ablend of polyethylene and a minor amount of a zinc-neutralized ioniccopolymer.

The laminate of this invention is advantageous not only because of theaforementioned characteristics but also because its layer ofpolyethylene is heat scalable to itself and the laminate itself isinexpensive to produce. The expensive nylon and adhesive layers can bekept thin and all of the materials comprising the laminate can bedirectly coextruded from a single special die as a film of three layers.The film can be used for example for making flat sheets, squeeze tubes,and parisons for blown bottles.

It is therefore an object of this invention to provide a laminated filmfor wrapping, packaging and related purposes.

It is another object of this invention to provide a laminated film thatis inexpensive, heat scalable and has a high resistance to permeation ofwater, moisture, oxygen, and various foodstuffs.

It is another object of the present invention to provide a laminatedfilm that is especially suitable for foodstuff-containing packages suchas bags or pouches subjectable to widely varying temperatures andconditions.

It is still another object of this invention to provide a laminated filmthat is especially suitable for making boil-in-the-bag pouches.

It is still another object of this invention to provide a laminated filmwhose materials are coextrudable from a single die in a single stepprocess.

DETAILED DESCRIPTION OF THE INVENTION The polyamide polymer utilized asa layer in the laminate of this invention is a nylon selected from thegroup consisting of polycaproamide, polyhexamethylene adipamide,polyhexamethylene sebacamide, polycaprylamide, polyundecanoamide andpolydodecanamide. These nylons are respectively commonly known asnylon-6, nylon-6,6, nylon 6,10, nylon 8, nylon 11, and nylon 12. Thepreferred polyamide is polycaproamide (nylon 6). The materials and theirmethods of preparation are well known in the art, and they arecommercially readily available. An example of a suitable polycaproarnideis manufactured by Allied Chemical Corporation and sold as Plaskon8201G.

The polyolefinic hydrocarbon polymer utilized in the laminate of thisinvention is selected from the group consisting of polyethylene,poly(ethylene co-vinyl acetate), poly(ethylene co-butene), poly(ethylenecohexene) and a blend of a major amount of polyethylene and a minoramount of a zinc-neutralized copolymer. Preferred polyolefinichydrocarbon polymers are polyethylene, and a blend of the polyethyleneand a zincneutralized ionic copolymer. Preferably, especially forlaminates subjected to high temperature environments, the polyethylenehas a density of from about 0.925 to about 0.970 grams per cc. Thispreferred range includes medium density polyethylenes which range up toabout 0.940 grams per cc. and high density polyethylenes which rangeabove about 0.940. Included in the 0.925 to 0.940 gram per c.c. rangeare some of the low density polyethylenes. Like the aforementionednylons, these polyolefins and their methods of preparation are wellknown in the art. An example of a suitable polyethylene of mediumdensity is produced by Union Carbide Company and sold as Bakelite DHDB4700, and a suitable one of high density is produced by PhillipsPetroleum Company and sold as Marlex 6009.

When polyethylene is used as a blend with a zincneutralized ioniccopolymer, it preferably is of medium density. The blend comprises amajor amount of polyethylene and a minor amount of the ionic copolymer.The amounts are based on' the weight of the blend. Preferably, the blendcomprises from about 51 to 95 percent polyethylene and from about 49 to5 percent ionic copolymer. More preferably, it comprises about 91percent polyethylene and about 9 percent ionic copolymer.

The ionic copolymers which can be used with polyethylene in theaforementioned blends and which can be used as adhesives arezinc-neutralized ionic copolymers of ethylene and an a,B-ethylenicallyunsaturated carboxylic acid. The ethylene component can be present in anamount of at least 50 mol percent based on the copolymer. The carboxylicacid monomer can be present in an amount of from about 0.2 to 25 molpercent, preferably 1 to 10 mol percent, based on the copolymer. Fromabout 10 to 90 percent of the acid groups of the acid are neutralized byzinc ions.

The a,B-ethylenically unsaturated carboxylic acids which can be employedto form the ionic copolymer adherent of this invention can be eithermono or dicarboxylic acids such as for example acrylic, methacrylic,ethacrylic, itaconic, maleic and fumaric acids, and, monoesters of thedicarboxylic acids such as methyl hydrogen maleate and methyl hydrogenfumarate. Of these acids, the preferred is methacrylic.

Ionic copolymers are well known and sold under the trademark Surlyn A,"registered by E. l. DuPont de Nemours & Company.

Examples of zinc-neutralized ionic copolymer Surlyns which can beutilized in the blends as adhesives of the laminates of this inventionare Surlyns A1650, about 40 percent neutralized, A 1652, about 23percent neutralized, AD807l, about 61 percent neutralized, A1800, about78 percent neutralized, and AD8004-4, about 68 percent neutralized.

Methods of preparing Surlyn ionic copolymers are believed disclosed inU.S. Pat. No. 3,264,272, filed on Apr. 8, 1963 and assigned to Dupont.That reference discloses that ionic copolymers can be employed asadhesives. Contrary to what is expected however, it has been found thationic copolymers in general cannot be employed as adhesives. loniccopolymers are quite selective as to the substrate materials to whichthey will adhere. Only certain ionic copolymers adhere to certainsubstratesf'flie reasons for this are not wholly clear. 1

Adhesion of the ionomers appears to depend on a variety of factors.One-of them is the type of metal ions used to neutralize the acid groupsof the copolymer. It has been found that'whereas sodium-neutralizedSurlyns do not satisfactorily adhere to nylon, zincneutralized Surlynsdo adhere well to nylon. As examples, Surlyns A1601 and AD8040, bothsodium neutralized, do not adhere satisfactorily to Nylon 6, butaforementioned zinc-neutralized Surlyns such as A1650 and A1800 adherevery well to nylon. For example, for a three-layered 1 5% inch diameterlaminated tube comprising respective layers of 5 mil Nylon 6 (AlliedPlaskon 820lG), 8 mil Surlyn A1650 or A1800, and 8 mil polyethylene ofany density, the adhesion of the Surlyns to Nylon 6 at dry temperaturesof about to 150F, was better than cohesion of the Surlyns themselves.The Nylon-6-Surlyn layers were virtually insepa-. rable by peel. Theyfailed to shear at loads of 1 l lb/iriclitb' 4T1blinEh. A

Another example of the fact that sodium-neutralized ionomers do notadhere satisfactorily to particular materials is that Surlyn AD8040 doesnot adhere to polyethylenes. Contrastingly, as shown in subsequentexamples, the zinc-neutralized ionomers enumerated in this invention doadhere to polyethylenes. Adhesion appears to depend on the densities ofthe polyethylenes, for it has been found that zinc-neutralized ionomersadhere fairly well to low density polyethylenes at temperatures belowabout F, but adhere decreasingly well at temperatures about 100F. On theother hand, the ionomers adhere well to high density polyethylenes,especially at temperatures above about 150F. This adhesion at hightemperatures makes zinc neutralized Surlyns and high densitypolyethylenes especially suitable for laminates for packages such asboil-in-the-bag pouches subjectable to high use temperatureenvironments. Adhesion also appears to depend to some extent on thetemperature at which the Surlyn-polyethylene laminates are extruded andperhaps to an even greater extent on the temperature to which thelaminate is subjected when it is in use. When zinc Surlyns A1650 andA1800 were respectively coextruded at a a temperature of about 450F witha low density polyethylene of about 0.918 grams per c.c., (Union CarbideDFD 3200 Nat 7), peel strengths of about 2 lb/inch or above wereobtained at dry temperatures below about 100F, about 1 lb/inch at aboutF, and below about 0.5 lb/inch at about 150F. But, when the samezinc-neutralized Surlyns were respectively coextruded at a temperatureof about 500F with a high density polyethylene of about 0.960 grams perc.c., (Marlex 6009), peel strengths obtained were about 3.75 lb/inch fordry temperatures of about 1 5 0 F, and about 2 1 b/ inch fdrtemperatures of about 212F.

The tests show that whereas adhesions to certain low densitypolyethylenes might be satisfactory for normal conditions of handlingand use, i.e., temperatures not much above about F., such adhesionstrengths would not be satisfactory for laminates for packages such asboil-in-the-bag pouches subjectable to higher temperature environments,e.g., hot or boiling water, where temperatures can be from about toabove about 212F. For such higher temperatures, satisfactory adhesionsare obtained using the preferred medium or high density polyethylenes.

Besides the previously mentioned factors, adhesion of Surlyns topolyethylene also appears to depend, at least to some extent, on thedegree or percentage of carboxylic acid'groups of the ionomer that areneutralized by zinc ions. It has been found that, in dry tests, Surlynswith low percentages of neutralization adhere better at low/or roomtemperatures, while those with higher percentages adhere better athigher temperatures For example, when bonded to high densitypolyethylenes, Surlyns such as AD8050 and A1650 respectively havingabout 28 percent and about 41 percent of their acid groupszinc-neutralized, each had peel strengths of about 8 lb/inch at drytemperatures of about 75F, while Surlyns such as AD807l and A1800respectively having about 61 and 78 percent neutralization, had peelstrengths of about 4.75 lb/inch at dry temperatures of about 75F.

It has also been found that adhesions of Surlyns having lesserpercentages of neutralization are apparently more affected by changes intemperature thanthose with greater percentages of neutralization. Peelstrengths of Surlyns with lower percentages of neutralization i.e.,below about 50 percent have been found to drop more sharply whentemperatures rise from 100F to about 212F, than Surlyns with higherpercentages of neutralization.

The overall thickness of the three-layered nylon- Surlyn-polyethylenelaminate of this invention can be anywhere within the range of fromabout 0.5 up to as high as about 200 mils. Usually, thickness is withinthe range of from about 0.5 to 21 mils and more usually it is from about1 to about 10 mils. Preferably, overall thickness ranges are from about1 to 5.6 mils, more preferably from about 1 to 4.6 mils. Overallthickness can depend on factors as the intended use, method ofmanufacture, and cost and availability of materials.

Thicknesses of the individual layers of the laminated film can varywithin the overall thickness range of up to about 200 mils, and can be,for the polyamide layer, from about 0.4 to 1.5 mil or more, preferablyfrom about 0.7 to 1.1 mil, for the ionic copolymer adhesive, fromthinnest measurable thicknesses to about 2 mils or more, preferably fromabout 0.1 to 1.5 mil., and, for the polyolefinic hydrocarbon layer, fromabout 0.8 to 3.5 mils or more, more preferably from about 1.5 to 3 mils,and most preferably about 2 mils.

The laminate of this invention can include more than three layers, solong as it includes at least the aforementioned polyamide, ioniccopolymer and polyolefin hydrocarbon polymer layers. For example, one ofthe ionic copolymers can be used to adhere another polyamide orpolyolefin hydrocarbon polymer to one or both sides of the laminate.Previously mentioned preferred materials, thicknesses and percentages byweight are also preferred for the laminate when it includes more thanonly the polyamide, ionic copolymer and polyolefin layers, except thatwhen the polyolefin layer is polyethylene, it can have a density of fromabout 0.910 to about 0.960 grams per c.c.

The method used to form the.at-least-three-layered laminate film of thisinvention can be any of the various known methods used for manufacturinglaminates. These include preforming each of the plies and adhering themtogether by suitable heat and pressure, or, preforming the nylon andpolyethylene plies, coating one or more of the plies with the adhesiveby, e.g., a roll or dispersion technique, and then subjecting the formedlaminate to suitable temperatures and pres sures. Though these methodsare satisfactory, a less time consuming,- less expensive and morepreferred method is to simultaneously coextrude the materials into theat least three-layered laminate by means of a single, special die. Thismethod can be employed because the materials used to form the laminateare easy to draw. The method is fast and economical because it is asingle-step, coextrusion process. Any of the various dies includingblown, cast, tube, or rod dies can be used, and the laminate formed canbe used for example in the form of flat sheets, bags or pouches, squeezetubes and parisons for blown bottles. Extrusion temperatures and speedscan vary depending upon various factors such as the types of materialsextruded, the number of layers to be formed, and the types of dies used.Suitable temperature and speeds for extruding for example, a threelayered 5 mil Nylon-6, 8 mil Surlyn A1800 and 8 mil Marlex 6009 laminatetube of about 1% inch diameter from a single step annular meltlamination die are from about 450 to 550 F at a take off rate of fromabout 5 to 500 fpm.

The laminates of this invention have a wide variety of usefulapplications. Among the most useful applications are commercial packagesfor foods such as smoked meats or frozen foods which can be heated orcooked while still in the package. The laminates of the invention havebeen used to form boil-in-the-bag pouches which have not delaminated andwhose one inch seal has not failed when cooked, uncovered, for up to 30minutes in boiling waters of up to 212F. The boil-in-the-bag pouches arenot suitable for baking or for use in ovens. Although the bags can bevented to allow escape of air and gases from the bag as it is beingheated, it is preferred that the bags not be vented for venting allowsthe possibility of juices and food material to escape from the bag andcollect on cookware.

I claim:

1. A laminate film which does not delaminate when subjected totemperatures ranging from about 120 to about 212F., comprising at leasta layer of polyamide polymer bonded by an adhesive to a layer of apolyolefinic hydrocarbon polymer, having a density of from about 0.925to 0.970 grams per c.c., said adhesive being an ionic copolymercomprised of at least 50 mo] percent ethylene based on the copolymer andabout 0.2 to 25 mol percent of an a, B -ethylenically unsaturatedcarboxylic acid based on the copolymer, said a, B -ethylene-acidcopolymer having from about 35 to 78 percent of its carboxylic acidgroups neutralized by zinc ions, and said polyolefinic hydrocarbonpolymer is a blend of a major amount of polyethylene and a minor amountof said ionic copolymer each of said polyamide and polyolefinichydrocarbon polymer layers not having been treated for adhesability.

2. The laminate of claim 1 wherein the polyethylene has a density offrom about 0.925 to 0.970 grams perc.c. and said carboxylic acid has upto two carboxyl groups.

3. The laminate of claim 2 wherein said polyamide polymer layer isselected from the group consisting of polycaproamide, polyhexamethyleneadipamide, polyhexamethylene sebacamide, polycaprylamide,polyundecamoamide and polydodecanamide.

4. The laminate of claim 3 wherein said polyamide polymer ispolycaproamide and said carboxylic acid is methacrylic acid.

5. The laminate of claim 1 wherein said' major amount of polyethylene isfrom about 51 to percent, and said minor amount of ionic copolymer isfrom about 49 to 5 percent based on the weight of the blend.

6. The laminate of claim 5 wherein said polyethylene has a density offrom about 0.925 to about 0.940 grams per c.c., said major amount isabout 91 percent and said minor amount is about 9 percent based on theweight of the blend.

7. The laminate of claim 1 wherein said laminate is obtained by asingle-pass coextrusion process.

8. The laminate of claim 5 wherein said laminate is obtained by asingle-pass coextrusion process.

2. The laminate of claim 1 wherein the polyethylene has a density offrom about 0.925 to 0.970 grams per c.c. and said carboxylic acid has upto two carboxyl groups.
 3. The laminate of claim 2 wherein saidpolyamide polymer layer is selected from the group consisting ofpolycaproamide, polyhexamethylene adipamide, polyhexamethylenesebacamide, polycaprylamide, polyundecamoamide and polydodecanamide. 4.The laminate of claim 3 wherein said polyamide polymer is polycaproamideand said carboxylic acid is methacrylic acid.
 5. The laminate of claim 1wherein said major amount of polyethylene is from about 51 to 95percent, and said minor amount of ionic copolymer is from about 49 to 5percent based on the weight of the blend.
 6. The laminate of claim 5wherein said polyethylene has a density of from about 0.925 to about0.940 grams per c.c., said major amount is about 91 percent and saidminor amount is about 9 percent based on the weight of the blend.
 7. Thelaminate of claim 1 wherein said laminate is obtained by a single-passcoextrusion process.
 8. The laminate of claim 5 wherein said laminate isobtained by a single-pass coextrusion process.